1. Field of the Invention
The present invention relates to a radiation image recording and reproducing method and a radiation image storage panel employed for the same. More particularly, the invention relates to a radiation image recording and reproducing method using a divalent europium activated complex halide stimulable phosphor and a radiation image storage panel employed for the same.
2. Description of the Prior Art
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiogrraphic film having an emulsion layer containing a sensitive silver salt material and an intensifying screen. As a method replacing the above-mentioned conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, has been paid much attention. The method involves steps of causing a stimulable phosphor to absorb a radiation having passed through an object or having radiated from an object; sequentially exciting (or scanning) the phosphor with an electromagnetic wave such as visible light or infrared rays (stimulating rays) to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably smaller dose, as compared with the conventional radiography. Accordingly, the radiation image recording and reproducing method is of great value, especially when the method is used for medical diagnosis.
There is proposed a divalent europium activated alkaline earth metal fluorohalide phosphor (M.sup.II FX:Eu.sup.2+, in which M.sup.II is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; and X is a halogen other than fluorine), as a stimulable phosphor employable in the radiation image recording reproducing method. The phosphor gives emission (stimulated emission) in the near ultraviolet region when excited with an electromagnetic wave such as visible light or infrared rays after absorption of a radiation such as X-rays.
As for a stimulable phosphor employable in the radiation image recording and reproducing method, almost no stimulable phosphor other than the divalent europium activated alkaline earth metal fluorohalide phosphor is known.
The present applicant discovered a novel divalent europium activated alkaline earth metal halide phosphor having the formula: EQU M.sup.II X.sub.2.aM.sup.II X'.sub.2 :xEu.sup.2+
in which M.sup.II is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; each of X and X' is at least one halogen selected from the group consisting of Cl, Br and I, and X.noteq.X'; and a and x are numbers satisfying the conditions of 0.1.ltoreq.a.ltoreq.10.0 and 0&lt;x.ltoreq.0.2, respectively, and applied for a patent with respect to said phosphor, a radiation image recording and reproducing method utilizing said phosphor and a radiation image storage panel employing said phosphor (U.S. patent application No. 660,987 and European Patent Application No. 84112417.5).
The novel divalent europium activated alkaline earth metal halide phosphor has been confirmed to have a crystal structure different from that of the aforementioned M.sup.II FX:Eu.sup.2+ phosphor on the basis of the X-ray diffraction patterns as described in the above application. This phosphor gives stimulated emission (peak wavelength: approx 405 nm) in the near ultraviolet to blue region when excited with an electromagnetic wave having a wavelength within the region of 450-1000 nm after exposure to a radiation such as X-rays, ultraviolet rays and cathode rays.
The radiation image recording and reproducing method using a radiation image storage panel which comprises the above-mentioned stimulable phosphor is very useful for obtaining a visible image as described above, and in this method, the sensitivity thereof is desired to be as high as possible. The sensitivity of the radiation image storage panel to a radiation generally increases as the luminance of stimulated emission of the phosphor employed for the panel increases. Accodingly, the stimulable phosphor employed for the panel is desired to show the luminance of stimulated emission as high as possible.